PROJECT SUMMARY Lung-restricted antibodies (LRA) against non-polymorphic lung self-antigens, collagen type V (COLV) and K- alpha1 tubulin (KAT), are present in over a third of patients undergoing lung transplantation. Since these self- antigens are expressed in all humans, pre-existing LRA in the recipient can bind to cognate antigens exposed in the freshly transplanted lungs. Indeed, LRA have emerged as the predominant risk factor for the development of primary graft dysfunction, the principal cause of early mortality following lung transplantation. In murine models, LRA are causally linked to severe lung allograft injury, and in patients they are associated with development of donor specific alloimmune responses. However, the mechanisms underlying their pathogenicity have not been elucidated. Given the high prevalence of LRA and their causal association with allograft injury, delineating these mechanisms should lead to relevant strategies to improve lung transplant survival. We have reported that LRA mediated allograft injury is associated with the deposition of complement and neutrophil infiltration. However, we do not understand the mechanisms by which LRA drive complement deposition or neutrophil recruitment, nor whether either is necessary for LRA-mediated injury. Since these lung self-antigens are located in the interstitial spaces, it is also unclear how circulating LRA gain access to these antigens. We have recently reported that spleen-derived Ly6ChighCCR2+ classical monocytes (CM) are recruited to the lungs following ischemia-reperfusion injury where they are necessary to open endothelial tight junctions by activating IL1? receptors (IL1R). Consistent with these findings, our preliminary data shows that syngeneic or allogeneic lung grafts from Il1r-/- donors are protected from LRA-mediated lung graft injury. We have also reported that Ly6ClowCCR2- pulmonary intravascular non-classical monocytes (NCM) are necessary for neutrophil recruitment into the donor lung. Our new preliminary data suggest that LRA cross-linked to their cognate antigens can enhance release of the neutrophil chemoattractant CXCL2 from human and murine NCM. Therefore, we hypothesize that extravasation of LRA into the lung interstitium requires IL1?-dependent opening of endothelial tight junctions by spleen-derived classical monocytes and, upon binding to cognate antigens in the interstitium, cross-linked LRA activate complement as well as donor-derived NCM to promote acute lung allograft injury. Accordingly, we will determine 1) whether splenic CM allow passage of LRA into the lungs by opening endothelial tight junctions after being educated in the spleen, 2) which complement pathway is responsible for LRA-mediated injury as well as the mechanisms of complement activation, and 3) whether toll-receptor independent activation of donor NCM by LRA leads to neutrophil recruitment and lung injury. Overall goal of these experiments is to provide novel insights into the biology of LRA and introduce clinically applicable strategies to inhibit lung injury following transplantation through pharmacological inhibition of CM- derived IL1? (Aim 1), complement (Aim 2), or deletion of donor NCM (Aim 3).